Hydrocarbon fuels have long been known to produce atmospheric pollutants when burned. The pollutants typically include nitric oxide (NO) and nitrogen dioxide (NO.sub.2), frequently grouped together as nitrogen oxides or NO.sub.x, unburned hydrocarbons (UHC), carbon monoxide (CO), and particulates, primarily carbon soot. NO.sub.x is of particular concern because of its role in forming ground level smog and acid rain and in depleting stratospheric ozone.
Hydrocarbon combustion forms NO.sub.x by several mechanisms. The high temperature reaction between atmospheric oxygen and atmospheric nitrogen, particularly at flame temperatures above about 1540.degree. C. (2800.degree. F.), forms NO.sub.x through the thermal or the Zeldovich mechanism ("thermal NO.sub.x "). The reaction between atmospheric nitrogen and hydrocarbon fuel fragments (CH.sub.i ), particularly under fuel-rich conditions, forms NO.sub.x through the prompt mechanism ("prompt NO.sub.x "). The reaction between nitrogen released from a nitrogen-containing fuel and atmospheric oxygen, particularly under fuel-lean conditions, forms NO.sub.x through the fuel-bound mechanism ("fuel-bound NO.sub.x "). Typically, atmospheric oxygen and nitrogen are readily available for the NO.sub.x -forming reactions in combustion air that is mixed with the fuel.
To limit NO.sub.x formation, many modern combustors burn fuel that has little or no nitrogen and operate at uniformly fuel-lean conditions. Burning low nitrogen fuel reduces or eliminates the formation of fuel-bound NO.sub.x. Operating under uniformly fuel-lean conditions, for example, by using a lean premixed/prevaporized system, reduces the formation of NO.sub.x by the thermal and prompt mechanisms. The excess air used to achieve fuel-lean conditions reduces thermal NO.sub.x formation by acting as a diluent to decrease flame temperatures. The excess air also decreases the concentration of CH.sub.i available to react with atmospheric nitrogen, thereby reducing the formation of prompt NO.sub.x. The amount of excess air needed to reduce thermal and prompt NO.sub.x formation can, however, cause the combustor to operate near its lean combustion limit, resulting in flame instability. Flame stability can be improved by supplementing the main flame with a pilot flame to ensure that the main flame remains lit, even at very lean conditions.
While fuel-lean combustion can successfully reduce NO.sub.x formation, there is a need to find ways of further reducing NO.sub.x production to meet increasingly stringent emission regulations. Therefore, what is needed in the industry is an improved fuel-lean, low NO.sub.x combustion system.